How Do the Axial and Equatorial Ligands Modulate the Reactivity of a Metal-Bound Terminal Oxidant? An Answer from the Hypochlorite Adduct of Iron(III) Porphyrin

Terminal oxidant adducts of transition-metal complexes have been known as not only the precursors of high-valent oxo species but also the reactive compounds that can catalyze various oxygenation reactions. While there have been various reports on the terminal oxidant adducts of peroxides, peracids, and iodosylarenes, the report of the hypochlorite adduct of metal complexes has been limited. We succeed in the preparation and characterization of bis-hypochlorite adducts of iron(III) porphyrin complexes having various electron-withdrawing substituents. The spectroscopic (ultraviolet–visible absorption, nuclear magnetic resonance, electron paramagnetic resonance, and extended X-ray absorption fine structure) studies and density functional theory calculations indicate that as the electron-donor effect of the porphyrin and axial ligands becomes stronger the Fe–OCl and O–Cl bonds of the iron-bound hypochlorite become weaker and thus the hypochlorite complex becomes more unstable. Furthermore, the kinetic analyses of the hypochlorite complexes for the epoxidation, chlorination, sulfoxidation, and hydrogen abstraction reactions indicate that as the Fe–OCl and O–Cl bonds become weaker the iron-bound hypochlorite becomes more reactive. These results allow us to answer the question how the axial and equatorial ligands control the reactivity of the iron-bound hypochlorite. The key is the electron migration from the fully filled OCl π*-orbitals to the vacant Fe 3dz2- and 4p-orbitals, which is essential for making the Fe–OCl coordination bond. An increase in the electron-donor effect of the ligand decreases the electron migration from the OCl π*-orbitals because of an increase in the energies of the Fe 3dz2- and 4p-orbitals. This change makes the electron density in the OCl π*-orbitals high, the Fe–O and O–Cl bonds weak, and the hypochlorite complex reactive. This would be a common mechanism for controlling the reactivity of other terminal oxidant adducts of transition-metal complexes.